The state of being surprisingly content with or in an unfamiliar circumstance.

"This is not something I'm used to, but it's chill."
"Mad chill man." (unspoken: This isn't what I might have done, but surprisingly I'm very content right now.)
"We're just chilling." (unspoken: We both just decided to sit down and do nothing, which is something neither of us normally do -- and we are very content)

When defining a chill person, one must take into account that a chill peson is not someone who is cool all of the time. An example of a chill perosn is someone who does whatever they feel like doing, not following crowds. Many, not all, but many have friends in all social groups and don't care who sees them hanging with who, whether it be the athletes, the skaters, the posers, or the nerds. They may play video games, but that is not to be frowned upon. They may not be the smartest around, or the best sweet talker, but they know how to have a good time by being who they are and being with whom they are friends, not dating the popular ones because they are popular themselves or picking on the nerds because their social status is hurt if they make friends with them. Most girls appreciate a man who stays as themselves and like the girl for who she is. Be chill like described here and you can be liked by anyone.

John: Look that guy's a fag. He hangs with the nerds.
Jim: Shut up. You're the fag. Jack's chill. Get it in your head that you can be friends with whom you want